Why my Eevee material with Alpha won’t show correctly
2 Solutions were found. 
#1 by Benny_G: Instead of using Alpha Blend for the Blend mode, use Alpha Hashed.
#2 by NewVisitor: Use the Principled Shader instead of the Diffuse BSDF Shader, plugging Alpha to Alpha from the Image node to the Principled Shader.

What caused the need for this fix, it's still unknown.